I had a go on one today and I can confirm the following facts:
1) It's weeeeeeeeeee. Surprisingly tiny actually.
2) it's a nice by of kit, though.
3) The remotes aren't exactly ergonomic. I didn't like them that much.
4) I kept sliding the right remote up because it wasn't clicked in properly when I thought it was.
5) The screen is good. Nice and bright.
6) The UI is ok.
7) I didn't like playing Mario on the wee screen. I much preferred the telly. Two player was ridiculously small on the small screen (smaller than an iPad) and wasn't really much fun. 4 player Mario Kart on the move would clearly be limited fun. It's just too sodding small.
I barely leave the house so gaming on the move isn't really for me. If you travel and don't have to, say, control a car then I can imagine it'll be quite groovy.
9) I didn't get to play anything that used HD Rumble but the Rumble felt quite strong even just playing Mario Kart.
Overall: It's not for me. At least not until it's got more big games I can play on the big screen for a big screen experience. Cool though.